Fin

ABSTRACT

A side fin ( 10 ) for a surfboard has a base edge portion ( 13 ) which in use juxtaposes the surfboard, a leading edge ( 14 ) and a trailing edge ( 15 ) which meet to define fin tip ( 16 ). Fin ( 10 ) has a substantially planar fin base portion ( 17 ) extending from base edge portion ( 13 ), and a substantially curved fin tip portion ( 18 ) extending from fin base portion ( 17 ) to fin tip ( 16 ). When the fin profile is viewed in the longitudinal direction of the surfboard i.e. from leading edge ( 14 ) to trailing edge ( 15 ), the profile of fin base portion ( 17 ) is substantially rectilinear and the profile of fin tip portion ( 18 ) is curved.

TECHNICAL FIELD

This invention relates to a fin.

The invention has particular but not exclusive application to fins forsurfboards and the like and for illustrative purposes reference will bemade throughout the specification to this application. However it is tobe understood that the fin of the present invention may be used in otherapplications including for example, sailboards, yachts, boats, ships andother keeled vessels.

BACKGROUND OF INVENTION

It is known for surfboards to have a central tail fin and a pair of sidefins. The side fins aim to increase manoeuvrability. It is also known todesign the side fins with a part circular profile when viewedlongitudinally.

SUMMARY OF INVENTION

The present invention aims to provide an alternative to known fins forsurfboards and the like.

This invention in one aspect resides broadly in a fin for a surfboard orthe like, the fin having a base edge portion which in use juxtaposes thesurfboard and a leading edge and a trailing edge which meet to define afin tip, the fin including:

a substantially planar fin base portion extending from the base edgeportion, and

a substantially curved fin tip portion extending from the fin baseportion to the fin tip:

wherein when the fin profile is viewed from the leading edge to thetrailing edge, the profile of the fin base portion is substantiallyrectilinear and the profile of the fin tip portion is curved.

The fin profile may have various configurations. In one preferredembodiment the radius of curvature of the profile of the fin tip portionprogressively decreases towards the fin tip. Alternatively the radius ofcurvature of the fin profile may progressively decrease towards the fintip. In one preferred embodiment the fin profile is substantiallyparabolic.

The fin may be integrally formed with the surfboard. However it ispreferred that the fin includes mounting means for mounting the fin to asurfboard, the mounting means being disposed at an angle to the fin baseportion such that when the fin is mounted to a surfboard at a sidethereof the fin base portion extends in an outwardly direction and thefin profile is concave with respect to the longitudinal axis of thesurfboard.

In another aspect this invention resides broadly in a surfboard having apair of side fins each having a base edge portion juxtaposing thesurfboard and a leading edge and a trailing edge which meet to define afin tip, the surface of the side fins including:

a substantially planar fin base portion extending from the base edgeportion, and

a substantially curved fin tip portion extending from the fin baseportion to the fin tip;

wherein when the fin profile is viewed along the longitudinal directionof the surfboard, the profile of the fin base portion is substantiallyrectilinear and the profile of the fin tip portion is curved;

the fin base portion of the side fins extending in an outwardlydirection from the surfboard and the fin profile of each side fin beingconcave with respect to the longitudinal axis of the surfboard.

DESCRIPTION OF DRAWINGS

In order that this invention may be more easily understood and put intopractical effect, reference will now be made to the accompanyingdrawings which illustrate a preferred embodiment of the invention,wherein:

FIGS. 1 to 3 are plan, side and end views respectively of the tailportion of a surfboard showing two side fins in accordance with thepresent invention and a central fin mounted thereto;

FIG. 4 is a perspective view of the tail portion of the surfboard seenin FIGS. 1 to 3;

FIGS. 5 to 7 are end, side and plan views respectively of a fin inaccordance with the present invention;

FIGS. 8 and 9 are perspective views of the fin seen in FIGS. 5 to 7, and

FIGS. 10A and 10B illustrate some principles of operation of surfboardswith curved side fins.

DESCRIPTION OF PREFERRED EMBODIMENT OF INVENTION

As can be seen in FIGS. 5 to 9, a side fin 10 for a surfboard has a baseedge portion 13 which in use juxtaposes the surfboard, a leading edge 14and a trailing edge 15 which meet to define fin tip 16. Fin 10 has asubstantially planar fin base portion 17 extending from base edgeportion 13, and a substantially curved fin tip portion 18 extending fromfin base portion 17 to fin tip 16.

When the fin profile is viewed in the longitudinal direction of thesurfboard ie from leading edge 14 to trailing edge 15, the profile offin base portion 17 is substantially rectilinear and the profile of fintip portion 18 is curved.

To most conveniently mount the fins to a surfboard (as seen in FIGS. 1to 4) a pair of spigots 19 are located on base edge portion 13 forconnection in well known manner in receiving mounts (not shown) locatedin surfboard 12. Surfboard 12 has a central spine fin 11 mountedrearwardly of a pair of laterally disposed side fins 10 in accordancewith the present invention.

As is best seen in FIG. 4, the mounting spigots 19 are disposed at anangle to fin base portion 17 such that when fins 10 are mounted tosurfboard 12 at a side thereof, fin base portion 17 extends in anoutwardly direction and the fin profile is concave with respect to thelongitudinal axis of surfboard 12.

When viewed in the longitudinal direction of the surfboard ie in thefore to aft direction or from the leading to trailing edge, the profileof the fin in accordance with the present invention can have a number ofconfigurations.

In one embodiment the profile of the fin base portion is rectilinear iethe fin base portion is planar, and the curvature of the fin tip portionis such that the radius of curvature of the profile of the fin tipportion progressively decreases towards the fin tip. Alternatively theprofile of the fin base portion can be slightly curved ie the fin baseportion is a slightly curved plane, and the radius of curvature of thefin profile progressively decreases towards the fin tip.

In another preferred embodiment the fin profile is substantiallyparabolic.

During experiments, experienced surfboard riders have found thatsurfboards having side fins in accordance with the present inventionhave improved performance characteristics over known surfboards. Theyhave reported improved grip or hold against the wave face, greater easeof manoeuvrability and a substantial improvement in speed generated.

The reasons for the improvements in performance of surfboards fittedwith side fins of the present invention over known surfboards are notfully understood. It is thought that it may be due, at least in part, tothe emphasised curvature toward the fin tip of the longitudinally viewedfin profile.

A hypothetical explanation of why this may be so is given with referenceto FIGS. 10A and 10B which respectively show different surfboardstravelling across a wave face, with one surfboard (FIG. 10A) having theside fins of the present invention and the other (FIG. 10B) having knowncircularly arced side fins.

Boards with circularly arced side fins balance advantages withdisadvantages and are effective only to a nominal degree. It is believedthat the improved quality of grip of the side fins of the presentinvention may derive from the emphasised curvature toward the fin tip ofthe longitudinally viewed fin profile.

As can be seen if FIG. 10, as a surfboard moves across a wave face therelative movement of the water with respect to the surfboard is upwardlysideways. The forces from the wave might overpower the surfer's downwardweighting and result in the surfboard moving away from the wave face. Asthis occurs the fins which are holding the surfboard from driftingsideways lift away from the wave face, thereby decreasing their abilityto resist the sideways momentum of the surfer, and so the surfboarddrifts sideways.

As can be seen in FIG. 10A, this effect is diminished in surfboardshaving the side fins of the present invention because there isadiminished tendency for the flow of water relative to the down-waveside fin to be deflected downwardly (which has the effect of generatinglift and imparting a clockwise rotation to the surfboard as viewedthereby increasing instability.) Moreover because of the emphasisedcurvature toward the fin tip, the up-wave side fin “holds” the wave andthus resists upward movement of the surfboard and the wave energy isthus effective in forcing the surfboard back up in a counterclockwisemanner thus offsetting any instabilities generated by the down-wave fin.Moreover it is believed that the greater the speed of the surfboardacross the wave face, the greater is the degree of hold on the wave faceby the up-wave fin.

On the other hand, as is seen in FIG. 10, the instability effectreferred to above is emphasised in surfboards having circularly arcedfins because there is a greater tendency for the flow of water relativeto the down-wave side fin to be deflected downwardly such that midwaythrough the upward lifting of the surfboard ie the clockwise rotation ofthe board about its longitudinal axis as viewed, the down-wave fingenerates lift and so increases instability. Moreover the regularcurvature of the up-wave side fin creates less hold on the wave facethan does the emphasised curvature toward the fin tip of the side fin ofthe present invention.

The side fins of the present invention have a number of advantages overknown side fins. Circularly arced side fins and flat sided side finsincrease grip to some extent but this is countered by a decrease inmanoeuvrability as well as an increase in drag through turning. Theincreased grip provided by the side fins of the present invention allowa more upright side profile to be used without the loss of controlusually arising as the side profile becomes more upright.

The side fins of the present invention provide an increase in grip ofthe surfboard on the wave face and a decrease in drag through turning,thus providing improved manoeuvrability and an increase in speedgenerated.

It will of course be realised that whilst the above has been given byway of an illustrative example of this invention, all such and othermodifications and variations hereto, as would be apparent to personsskilled in the art, are deemed to fall within the broad scope and ambitof this invention as is herein set forth.

Thus it will be apparent that the invention can be utilisedadvantageously in other water craft including sailboards as well asships, yachts and sail boats when travelling across the direction ofwave motion.

What is claimed is:
 1. A fin for a surfboard, said fin having a baseedge portion which in use juxtaposes the surfboard and a leading edgeand a trailing edge which meet to define a fin tip, said fin including:a substantially planar fin base portion extending from said base edgeportion, and a substantially curved fin tip portion extending from saidbase portion to said fin tip; wherein when the fin profile is viewedfrom the leading edge to the trailing edge, the profile of said fin baseportion is substantially rectilinear and the profile of said fin tipportion is curved.
 2. A fin as claimed in claim 1, wherein the radius ofcurvature of said profile of said fin tip portion progressivelydecreases towards the fin tip.
 3. A fin as claimed in claim 2, whereinsaid profile of said fin tip portion is substantially parabolic.
 4. Afin as claimed in claim 1, wherein said fin profile is substantiallyparabolic.
 5. A fin as claimed in claim 1, and including mounting meansfor mounting the fin to a surfboard, said mounting means being disposedat an angle to said fin base portion such that when the fin is mountedto a surfboard at a side thereof said fin base portion extends in anoutwardly direction and the fin profile is concave with respect to thelongitudinal axis of the surfboard.
 6. A fin for a surfboard comprising,a base edge portion which in use juxtaposes the surfboard and a leadingedge and a trailing edge which meet to define a fin tip; one or morespigot connected to said base edge, said spigot for connecting said finto said surfboard; a substantially planar fin base portion extendingfrom said base edge portion, and a substantially curved fin tip portionextending from said base portion to said fin tip; wherein when the finprofile is viewed from the leading edge to the trailing edge, theprofile of said fin base portion is substantially rectilinear and theprofile of said fin tip portion is curved.
 7. A fin for a water craftcomprising, a base edge portion which in use juxtaposes the water craftand a leading edge and a trailing edge which meet to define a fin tip; asubstantially planar fin base portion extending from said base edgeportion, and a substantially curved fin tip portion extending from saidbase portion to said fin tip; wherein when the fin profile is viewedfrom the leading edge to the trailing edge, the profile of said fin baseportion is substantially rectilinear and the profile of said fin tipportion is curved.
 8. The fin of claim 7, wherein the water craft is asurfboard, sailboard, ship, yacht or sail boat.